Hi,
so I came up with a melody and then started messing around with the rhytm guitar - I just recorded the melody in a loop and tried various power chord progressions under it, without really analyzing it much beforehand. I found one that I like very much for how it alters or complements the feel of the melody.
Thing is, the most prominent tone became a major sixth of the chord under it and upon playing it to my guitar teacher, he said that he thinks this tone is standing out too much for it not to be a chord tone.
So I need to determine whether I like it because my ear isn't that developed yet, or whether you think it actually works.
It's going to be a part of a melodethish song, if genre specification is of any relevance here...

Chances are your teacher is probably an inexperienced noob. I haven't looked at your file because I don't have Guitar Pro, but hearing a Major 6th the way you described it. I would immediately be thinking in basic terms, either a minor chord in first inversion, or a major chord in second version, depending on what else I hear.

uhm sorry, gonna fix those right away and thanks ("accord tone" should have been OK as well, so I wasn't that far off with this one)

And regarding the chord tone, I meant that the chord underneath should contain that tone, if it was a triad based of the same root note as the power chord is. So a third would be regarded as a chord note as well, even though it's not contained in the power chord.

melodethish - an a seems to have slipped me(even though I think I've seen it written as "melodeth" before) - supposed to be a melodeath song, but I'm not going to be obssessed with squeezing in the genre

I don't see why you should only use chord tones. That sixth just makes it a C6 and I think it's used a lot. If you wanted to change it, you could change that C chord to A. But I think it sounds good this way, you don't need to change it. I don't think that sounded dissonant at all. And if you think this chord progression feels the best, use it. It's about what sounds best to you because it's your song.

Based on the fact that you've got a C and G in your rhythm guitar and an A in your melody, and based also on the fact that the melody doesn't sound at all out of place, I'd say it's more likely that you're indicating an Am7 chord with no 5th in first inversion (3rd in the bass) than a C6 chord. In order for a C6 chord to make sense, you'd at least need an E in the chord to give it a third - it's much more natural to have an Am chord with the 5th (E) missing than a C6 chord with the 3rd (E) missing. Anyway, just tell your teacher it's an Am7 chord and that the A fits just fine.

^ Except it doesn't resolve down to G while the C is still playing, it resolves down only after the measure ends and the D chord comes in, at which point the G it resolves to isn't a chord tone either. Not an appoggiatura. And although the root-5 relationship is strong, if there's a potential root-3 relationship to be found in the chord, that will typically trump any apparent root-5 relationship. Furthermore, the Am7 "sound" doesn't have to be brought out, it just has to keep the G chord tone from sounding out of place, which it doesn't if you play an Am7 chord. Minor 7 chords are used all the time as predominants of bVII chords, even though it doesn't sound like a Minor 7 chord in the jazzy sense. I'll admit that neither chord is wrong, but the Am7 seems to make more sense from a more traditional approach, which is what this sounds like it's going for.

Christ... After reading this thread I guess it's a question of relationship between rhythm guitar and melody. I don't know if it sits on an A over a C and G in the rythym guitar but not every note in the melody has to fit with the chords. Otherwise you'd have melodies consisting of arpeggios and would end up sounding angular and/or boring. Tension and release are what makes melodies interesting.

Also, when using roman numerals in reference to chord qualities, I think it's generally accepted that when you're talking in minor (using i instead of I) that the scale is implied. For example, saying
e: i vi VII i
Implies:
Em Cm D Em because the 6th is flatted from the parallel major, but a minor quality and in natural minor the 7th is " " " " ", but a major quality.

Also, when using roman numerals in reference to chord qualities, I think it's generally accepted that when you're talking in minor (using i instead of I) that the scale is implied. For example, saying
e: i vi VII i
Implies:
Em Cm D Em because the 6th is flatted from the parallel major, but a minor quality and in natural minor the 7th is " " " " ", but a major quality.

Yeah I've seen it both ways. I prefer to specifiy the roman numerals in relation to the major scale even when in a minor key. In the same way that one does so for the arabic numerals that reperesent the scale degrees.

The reason I do this is because there are times when I might not stick to the diatonic chords which can create an issue...
if you are going to assume that when the tonic is minor then VI refers to the chord built off a minor sixth root how would you represent a chord built off the major sixth root?

For example if you were in Em and VI represents C major how do you represent a C#m chord with a roman numeral?

The solution I use is to use the roman numerals in reference the major scale. So bVI always refers to the chord built on the minor sixth and VI always refers to the chord built on the major sixth scale degree.

I have seen authors use bVI and bVI and bIII in a minor key in the same way that I do. There is not a consensus either way. I just chose to use the method that made the most logical sense to me.

For example if you were in Em and VI represents C major how do you represent a C#m chord with a roman numeral?

I understand the rest of your post and it makes sense and it's fine.
If you're in Em and theres a C#m chord, that would suggest some sort of modulation (as C#m has a G# in it... Kinda the anti-Em =P)

So in doing a Roman Numeral analysis of something, you have to look at where the chords go and label them appropriately. For example a C#m might indicate a modulation to B Major, especially if the chord after C#m is an F#Maj.
You'd go along as:
e: i ... ... ... B: ii V I ... ... ...
(With the B bracketed) under the music

I understand the rest of your post and it makes sense and it's fine.
If you're in Em and theres a C#m chord, that would suggest some sort of modulation (as C#m has a G# in it... Kinda the anti-Em =P)

So in doing a Roman Numeral analysis of something, you have to look at where the chords go and label them appropriately. For example a C#m might indicate a modulation to B Major, especially if the chord after C#m is an F#Maj.
You'd go along as:
e: i ... ... ... B: ii V I ... ... ...
(With the B bracketed) under the music

Not necessarily. For example if you look at Light My Fire by The Doors. The verse progression is Am - F#m. There are only two chords in the verse and it stays in one key, A minor.

Another example: Progression Am - G - F#m - F (- E). It stays in A minor all the time.